Heparin induces α-synuclein to form new fibril polymorphs with attenuated neuropathology

α-Synuclein (α-syn), as a primary pathogenic protein in Parkinson’s disease (PD) and other synucleinopathies, exhibits a high potential to form polymorphic fibrils. Chemical ligands have been found to involve in the assembly of α-syn fibrils in patients’ brains. However, how ligands influence the fibril polymorphism remains vague. Here, we report the near-atomic structures of α-syn fibrils in complex with heparin, a representative glycosaminoglycan (GAG), determined by cryo-electron microscopy (cryo-EM). The structures demonstrate that the presence of heparin completely alters the fibril assembly via rearranging the charge interactions of α-syn both at the intramolecular and the inter-protofilamental levels, which leads to the generation of four fibril polymorphs. Remarkably, in one of the fibril polymorphs, α-syn folds into a distinctive conformation that has not been observed previously. Moreover, the heparin-α-syn complex fibrils exhibit diminished neuropathology in primary neurons. Our work provides the structural mechanism for how heparin determines the assembly of α-syn fibrils, and emphasizes the important role of biological polymers in the conformational selection and neuropathology regulation of amyloid fibrils.